1. Field of the Invention
This invention relates generally to cryogenic storage tanks, and more particularly relates to a resilient, insulating blanket useable in such a tank and having a flexible, high tensile strength facing bonded to at least one side thereof to increase the strength of the blanket, and to a method of making the blanket.
2. Description of the Prior Art
One of the problems associated with the use of granular, insulating material, such as perlite, in the insulating space between the spaced walls of the inner and outer storage vessels of cryogenic storage tanks has been that of compaction, crushing, and attrition of the granular insulating material due to thermal expansion and contraction of the inner vessel. To overcome this problem, resilient, insulating blankets, which retain their compressive resilience at low temperatures, have been installed in the insulating space of the tanks to compensate for the aforementioned expansion and contraction of the inner and outer vessels. Examples of such arrangements are disclosed in the U.S. Pat. No. 3,147,878 Wissmiller and U.S. Pat. No. 3,612,332 Clapp.
While resilient, insulating blankets having low temperature compressive resiliency have heretofore been used to overcome the aforementioned problems, difficulties were sometimes encountered in the use of such blankets due to local tearing and rupturing of the blankets as a result of the large vertical drag forces imposed on the blankets from the granular insulating material. In other words, the recognized arching characteristic of the granular material, aggravated by settlement due to the changes in the dimensions of the insulating space of the tank, as the temperature of the tank went from ambient to cryogenic, and vice-versa, sometimes resulted in the imposition of vertical drag forces on the blanket which exceeded its tensile strength. When this occurred, the aforementioned local tearing or rupturing of the blanket took place, and portions of the blanket collapsed toward the bottom of the insulating space. Consequently, the capability of the tank to maintain cryogenic temperatures was substantially impaired, or lost, because the attrition rate of the granular insulating material substantially increased. The resulting perlite settlement also caused voids at the top of the shell and these voids, in turn, caused undesirable cooling of the outer tank walls.
In addition to the described problem, a separate problem was involved in installing the blankets. Several common methods of installation were to position a roll of glass cloth and one or two rolls of matted glass fibers constituting resilient blanket, either inside the tank roof above the annular space between the tank walls to be insulated, or outside the tank walls at the base of the tank walls to be insulated. As the glass cloth and blanket were unrolled simultaneously and lowered or raised into the annular space they were pinned together. This was tedious, time consuming and not enjoyable labor. Furthermore, as or after the pinned together material was installed, the blanket would sometimes tear in half and pull loose from the glass cloth, after the perlite was added, and thence fall to the bottom of the annular space. Since visability in the annular space was generally very poor the repair of a broken blanket was extremely uncomfortable and tedious.